Frequently for the energy supply of electronic apparatuses and installations and/or for the control of drives, processes etc. there are needed regulated or also adjustable direct voltages and currents. In the case of relative great powers there are used for the generation of direct voltages and currents controllable rectifiers in which thyristors are used. Thyristors are controllable valves which are also blockable in pass direction, in which, however, with voltage applied in this direction the time point of the current onset can be determined by an ignition process. The ignition of a thyristor occurs as a rule through an ignition impulse which is delivered from an ignition apparatus, the time point of the ignition determining the magnitude of the voltage or of the current, which is delivered to the user.
An ignition stage for a one-phase bridge rectifier operates as follows. The time onset of the ignition impulses (ignition angle) referred to the beginning of half period of the main alternating voltage (voltage zero passage) determines the height of the direct voltage. An ignition angle of 0.degree. means electrically, for example, in the case of ohmic-inductive load of the rectifier full direct voltage, an ignition angle of 180.degree. electrically means, in contrast, zero direct voltage . The time-defined ignition impulses are generated there as follows: An impulse generator in the form of a synchronizing stage forms from the main alternating voltage synchronizing impulses which correspond exactly to the zero passages of the alternating voltage. With these impulses there is triggered a sawtooth generator which delivers a voltage rising in ramp form. In a comparison circuit this voltage is compared with an external voltage which represents the ignition angle of the apparatus and thereby the desired value of the direct voltage. This desired magnitude is normally generated by a master controller. In the case of equality of the two voltages the comparison circuit triggers the generation of a drive impulse for the thyristor. Since in a one-phase bridge rectifier each half-wave of the main voltage is rectified by different thyristors, there is engaged on the outlet side of the comparison circuit a further stage for channel separation. This evaluates the positive and negative half-wave of the mains voltage and gives out every 180.degree. electrically an impulse to the corresponding thryistors.
For complex control, monitoring and regulating problems, in which a large amount of data are to be processed, in installations or apparatuses there are frequently used computers (mini- or microcomputers), since these present considerable advantages over the use of permanently wired analog circuits, for example in respect to the flexibility of the parameter changes, in respect to the circuitry expenditure and the costs. In such apparatuses or installations the desired values of needed direct voltages and currents and thereby the ignition angle of thyristors are calculated by the computer and are present, therefore, in the digital form. Nevertheless, hitherto the above-described ignition apparatuses for thyristors have been used in such installations and apparatuses, in which for the generation of the needed analog desired value for the ignition apparatus digital-analog transducers were used which transform the binary number (desired value) prescribed by the computer into a corresponding voltage.
This method has, to be sure, proved itself for a long time, but it is affected with several disadvantages. Thus, for an accurate digital-analog conversion there is required a very precise reference magnitude (either current or voltage), which must have as low as possible a temperature coefficient. Such component are available, to be sure, but they bring about relatively high costs. Nevertheless, even with use of very accurate D/A converters, in consequence of deficient long-time stability there can occur transmission errors which have a negative effect on the quality of the regulation.
The component stages of the ignition apparatus of the thyristors engaged on the outlet side of the D/A converter are constructed as a rule with operational amplifiers. Here, too, it is necessary to reckon with a certain temperature drift, which has a negative effect on the regulation. In order to catch up certain exemplary scatters of active or passive components, and in order to set in certain maximum ignition angles, in this stage, as a rule, a balancing is necessary.
Even in the use of analog, integrated circuits which already contain a large part of the components needed for an ignition apparatus, it is to be considered that passive components are necessary as external circuitry for an integrated circuit, (for example, capacitors and resistors for the generation of the ramp function), which by reason of imprecision and exemplary scatters again decrease the otherwise good properties of the integrated circuit. The use of analog integrated circuit has, to be sure, lowered the manufacturing costs of the ignition apparatus, but the still necessary passive components hardly allow cost reductions. Furthermore, as a rule, here, too, a balancing is required.
Similar problems arise, too, in circuits with power transistors which are used as adjusting members for the generation of transformation of direct voltages. Of these so-called switching regulators there exist many variants, for example, blocking transformers, one-beat or push-pull flux transformers or also bridge transformers, common to all of which is the working principle that is based on the raising of the operating frequency and the driving of power transistors as rapid switches.
A switching circuit, for example, operates as follows: The mains voltage is rectified, converted by the switching regulator into a square wave voltage and transformed to the desired voltage level. The square wave voltage is then again rectified.
Switching regulators are operated at fixed frequencies in the range from 1 kHz to about 100 kHz. The adjustment or regulation of current and voltage occurs over the impulse width of the square wave signal that is delivered from an analog regulator, and with which the power transistors are driven.
Similarly as in the operation of thryistors, also in this case there must be generated a signal precisely defined in the time and synchronized with a firmly prescribed frequency. In apparatuses or installations in which such switching regulators are used, and in which the regulation and prescription of the desired values occurs from a digital computer, there is used as a rule, the same principle of the generation of the control impulses as in the drive of thryistors, namely use of the component groups D/A converter--impulse generator for the generation of synchronizing impulses--actual value giver in the form of a ramp generator--comparison circuit.
Although in switching regulators integrated circuits are being used to an increasing degree, which contain nearly all the required functions, here, too, further passive components must be used, the disadvantages of which were already mentioned above.
The invention proceeds from a device from the generation of regulated and/or adjustable direct voltages or direct currents with use of semiconductor elements in the form of thyristors or power transistors, with an impulse generator for the generation of sychronizing impulses of prescribed frequency, with an actual-value giver controlled according to provision of the impulse generator, with a desired-value giver that delivers desired values which are a measure for the required direct voltage or for the required direct current, and with a comparison circuit which compares the output of the actual-value giver with the desired values and on equality delivers in each case a drive signal for the semiconductor element.
Underlying the invention is the problem of creating a device of the aforementioned type, which with digital desired-value prescription, is simpler in construction than the known circuits and assures a higher quality of the regulation.